This invention relates generally to imaging devices and more particularly to imaging devices with a plurality of imagers that provide sequential images that are overlaid to form a composite image.
Imaging devices often utilize a first color to produce an image, portions of which are desired to be highlighted using a second color. In order to produce the desired results the imaging device must precisely register the highlight color image with the first image. Highlight color image registration is often challenging. It is often the case that a highlight printer is designed as a retrofit of a monochromatic engine in which the quality of the motion of the photoreceptor is only good enough to limit the banding to a tolerable level. The monochromatic image is typically laid down at a constant rate of lines per unit time. If the second imager is also caused to write at a constant rate, serious errors in color to color registration may occur.
In single pass electrophotographic printers having more than one process station which provide sequential images to form a composite image, critical control of the registration of each of the sequenced images is required. This is also true in multiple pass color printers, which produce sequential developed images superimposed onto a photoreceptor belt for charging with toner to form a multi-color image. Failure to achieve registration of the images yields printed copies in which the color separations forming the images are misaligned. This condition is generally obvious upon viewing of the copy; as such copies usually exhibit fuzzy color separation between color patches, bleeding and/or other errors, which make such copies unsuitable for intended uses.
A typical highlight color reproduction machine records successive electrostatic latent images on the photoconductive surface. One latent image is usually developed with black toner. The other latent image is developed with color highlighting toner, e.g. red toner. These developed toner powder images are transferred to a sheet to form a color-highlighted document. When combined, these developed images form an image corresponding to the entire original document being printed. Such color highlighting reproduction machine can be of the so-called single-pass variety, where the color separations are generated sequentially by separate imaging and toning stations, or of the so-called multiple-pass variety, where the separations are generated by a single imaging station in subsequent passes of the photoreceptor and are alternatively toned by appropriate toning stations. A particular variety of single-pass highlight color reproduction machines using tri-level printing have also been developed. Tri-level electro-statographic printing is described in greater detail in U.S. Pat. No. 4,078,929. As described in this patent, the latent image is developed with toner particles of first and second colors simultaneously. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged.
Another type of color reproduction machine which may produce highlight color copies initially charges the photoconductive member. Thereafter, the charged portion of the photoconductive member is discharged to form an electrostatic latent image thereon. The latent image is subsequently developed with black toner particles. The photoconductive member is then recharged and image wise exposed to record the highlight color portions of the latent image thereon. A highlight latent image is then developed with toner particles of a color other than black, e.g. red, and then developed to form the highlight latent image. Thereafter, both toner powder images are transferred to a sheet and subsequently fused thereto to form a highlight color document.
The operation of highlight and color printers is well known and is described in greater detail in U.S. Pat. Nos. 5,113,202; 5,208,636; 5,281,999; and 5,394,223, the disclosures of which are hereby incorporated herein by this reference.
A hybrid reflex writing printer is described in commonly-owned U.S. patent application Ser. No. 10/909,075, which is incorporated by reference herein.
A simple, relatively inexpensive, and accurate approach to register latent images superimposed in such printing systems has been a goal in the design, manufacture and use of electrophotographic printers. This need has been particularly recognized in the color and highlight color portion of electro-photography. The need to provide accurate and inexpensive registration has become more acute, as the demand for high quality, relatively inexpensive color images has increased.
The disclosed imaging device utilizes a second imager for forming the highlight latent image at a time following the forming of the first latent image that accounts for irregularities in the movement of the photoreceptor belt between the first imager and the second imager. If the second imager is an LED bar as disclosed herein, one can take advantage of its ability to fire a line of data whenever it is most appropriate for color registration.
According to one aspect of the disclosure, an imaging device and method are provided for producing multicolor images from image data containing data representing an image of a first color and an image of a second color to be registered relative to the image of the first color onto a substrate by transferring colorants of the first and second colors to the substrate. The imaging device includes a first imager configured to generate an output corresponding to the image of the first color at a first exposure station. A second imager is configured to generate an output corresponding to the image of the second color at a second exposure station. A photoreceptor belt is configured to pass the first imager and the second imager. A photoreceptor drive system is coupled to the photoreceptor belt to drive the photoreceptor belt in a process path past the first and second imagers in a process direction. An encoder generating encoder pulses is coupled to the photoreceptor drive system. The second imager is displaced along the process path from the first imager by a displacement corresponding to a nominal number of the encoder pulses. A controller is coupled to receive the encoder pulses. The controller determines an actual machine clock period based on a time between successive encoder pulses. The controller generates a simulated machine clock signal based on a running average of a plurality of the actual machine clock periods. The controller uses the simulated machine clock signal to count up to the nominal number following firing of the first imager for a given scanline of the image data to determine a target time for firing the second imager for the given scanline.
Additional features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.
These figures merely illustrate the disclosed methods and apparatus and are not intended to exactly indicate relative size and dimensions of the device or components thereof.